The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Stimulation of subterranean hydrocarbon reservoirs and injector wells are widely carried out in the oilfield services industry. The most common techniques, including matrix acidizing, hydraulic fracturing, acid-fracturing, sand control, enhanced oil-recovery, etc. use aqueous fluids to impact hydrocarbon productivity. However, the majority of the aqueous fluids are executed with little knowledge of or consideration for the wettability (water-wet or oil-wet) or the partial water/oil saturation of the rock being treated. In fact, a large number of impediments to production can be attributed to improper formation-wettability.
Water-blocks often result from increased water-production occurring through any of the well-known water-problem types. Water-blocks in the formation are one of the most well-known formation damage mechanisms that diminish hydrocarbon productivity. However, many water-control and water-block remediation treatments are not designed for long-term formation wettability. Issues of wettability are particularly critical in the porous matrix of sandstone reservoirs, where the pore throat diameters are often very small (<10 μm) and thin water-wet/oil-wet coatings of the formation can constrain those pore throat diameters even further. In matrix acidizing, reactive acid fluids are intended to dissolve damaging mineral deposits or other induced particulate damage that may be, at the time of treatment, oil-wet, leaving the immiscible aqueous acid incapable of contacting a large portion of the damaging minerals. In acidizing fluids, “mutual solvents” are often added to temporarily reduce the interfacial tension between the acid and hydrocarbon; amphiphilic surfactants are often added to the acid to leave the formation water-wet enabling more efficient acid/mineral contact. In hydraulic fracturing, formation-wettability is generally considered less important because a) the exposed formation surface area is greatly increased due to the formation of a large fracture and 2) because the hydraulic fracturing fluid is not needed to dissolve mineral damage, consideration of the wettability of the formation adjacent to the fracture is generally not taken into consideration.
However, a large volume of aqueous fracturing fluid leaks off into the formation through the fracture-faces and as a result of its immiscibility with hydrocarbon can be very slow to return to the surface due to the sandstone being preferentially water-wet. A major failure to achieve expected stimulation from a fracturing treatment is through imbibement of water in the formation and proppant pack that is detrimental to hydrocarbon production. The same types of amphiphilic surfactants that are used in acidizing fluids are often added to fracturing fluids for reduced interfacial tension and wettability modification. However, these same surfactants have been used for many decades in stimulation fluids and their mechanism of action is ill-understood and is rarely tailored to formation or fluid properties. These surfactants have been widely proposed as additives for fracturing fluids that will absorb to solid substrates (such as formation or proppant) rendering those surfaces hydrophobic. Capillary pressure in the matrix or pack treated by these surfactants is impacted by both altered wettability and reduced surface tension after their absorption. However, surfactants do not form a persistent or covalent coating on the surface of the formation or proppant and offer only a temporary modification to the formation wettability. They are often swept from the formation surface with aqueous treatment fluid flowback or with the onset of hydrocarbon production.
Improved methods to resolve wettability issues inside the proppant pack of a fracture generated during a hydraulic fracturing treatment are needed. Though the porosities of propped-fractures are much higher than a sandstone matrix/formation, and issues of wettability are less critical in affecting production through the proppant pack. However, certain properties of the proppant pack and fluid filter-cakes could impact the wettability of the proppant pack. For example, resin-coated proppants are used frequently for proppant-flowback control and are generally oil-wet. However, resin-coated proppants have a number of incompatibilities with a number of carrier fluids due to the polymer coating-chemistry and certain fines that are formed in the manufacturing of the resin-coated proppant.
In general, the understanding of sandstone and other formation wettability has greatly advanced in recent years. However, the identity and chemical properties of additives to affect and control wettability (including the chemistry of surfactant additives to stimulation fluids) has not changed. Methods and compositions to tailor wettability properties are desirable.